1. Field of the Invention
This invention relates to an apparatus for observing or photographing the fundus of an eye, and in particular to improvements in an optical system including an objective lens common to the illuminating system and the observing and photographing system thereof.
2. Description of the Prior Art
In an eye fundus observing and photographing apparatus of the type in which an objective lens is common to the illuminating optical system and the observation optical system, if the the illuminating light reflected by the objective lens mixes in the observing and photographing system, the quality of the image of the fundus of an eye by the observing and photographing system will be remarkably deteriorated. Therefore, various contrivances have heretofore been made for the light reflected by the objective lens. In recent years, various objective lenses comprised of two or more lens groups have been proposed to meet the desire to make the photographing angle of view wider. If the number of lenses and the number of component groups are increased, the refractive power of the objective lens can be diffused and therefore the angle of view can be made wider, but the elimination of the ghost light by the reflection on the lens surface becomes difficult.
As an idea for solving such a problem, for example, an objective lens comprising two positive meniscus lenses disposed with their convex surfaces facing each other is disclosed in German Utility Model Publication No. 1694082. Here, the cornea of the eye to be examined and the observation aperture on the examiner side are in conjugate relation with the entire objective lens, the positive meniscus lens adjacent to the eye to be examined has its concave surface facing the eye to be examined, the center of curvature thereof is coincident with a position conjugate with the observation aperture with respect to the entire objective lens, the positive meniscus lens adjacent to the observation aperture side has its concave surface facing the observation aperture side and the center of curvature thereof is coincident with the observation aperture. An example in which one or both of the two positive meniscus lenses are constructed as a cemented lens is also shown in said German utility model publication. In this objective lens, however, the lens surfaces on which the elimination of the reflected light is taken into consideration are only the lens surface which is adjacent to the eye to be examined and the lens surface which is adjacent to the observation aperture. With regard to the convex surfaces facing each other, said German utility model publication mentions that anti-reflection films are provided thereon, but discloses no means for eliminating the reflected light, including the cemented surface of each lens. The reflectance of the retina of the eye fundus is as low as several percent and an intense strobo is used for the photographing thereof and therefore, the reflected light even on the lens surfaces provided with anti-reflection films or the cemented surfaces cannot be neglected but reduces the quality of the image and thus, the objective lens disclosed in the aforementioned German utility model publication has lacked practical utility.
An example of an objective lens comprising two positive meniscus lenses having their convex surfaces facing each other is shown also in U.S. Pat. No. 3,914,032. In this objective lens, the constructions of the concave surface adjacent to the eye to be examined and the concave surface adjacent to the observation aperture are the same as those in the aforementioned German utility model publication, but this U.S. patent further shows a construction in which the light reflected by the cemented surface provided in the positive meniscus lens adjacent to the eye to be examined is eliminated with the light reflected by the convex surface of this positive meniscus lens. That is, in this patent, there is disclosed a construction in which, of the refracting surfaces other than the first and last surfaces of the objective lens as counted from the eye to be examined, the radius of curvature of that surface having its center of curvature on the eye to be examined side is 80% or less of the combined focal length of the objective lens and in order to eliminate the light reflected by the opposed surfaces of the two positive meniscus lenses and the cemented surface provided in the lens adjacent to the eye to be examined, a light-intercepting material is provided on the vertex of the lens surface of each lens which is adjacent to the observation aperture.
Here, the condition under which the radius of curvature of that refracting surface having its center of curvature on the eye to be examined side is 80% or less of the combined focal length of the objective lens has been a condition necessary to reduce the size of the image of the observation aperture by the reflected light on this surface and to eliminate the light reflected by the light-intercepting material provided on the lens surface. However, limiting the radius of curvature to a small value by such a condition, not only tends to make the distribution of refractive power onto this surface excessively great and adversely affect the aberration correction, but also makes it difficult to secure the effective diameter of the lens and thus has been disadvantageous to making the angle of view wider.